1. Field of the Invention
The present invention relates to illuminating optical fibers and manufacturing processes to produce optical fibers having patterned light diffusion sites along the length of the fibers.
2. Information Disclosure Statement
The radiation emitted by a laser beam source can be coupled into an optical fiber of suitable dimensions and optical properties wherein the light can be transported with no significant losses over very long distances. Today""s state of the art fibers have found broad application in the fields of telecommunication, optical inspection, medical therapy, laser applications and many more. The fabrication processes are well understood and optical fibers are manufactured in large quantities at high quality providing lifetimes up to over one million hours.
Optical fibers rely on total internal reflection at the interface between the fiber core and the surrounding cladding to contain the light within the core of the fiber. The light guiding effect occurs in optical fibers where cores are much larger than the wavelength of the incident light. For light guiding to occur, the refractive index of the fiber cladding must be lower than the refractive index of the fiber core. A light ray incident to the fiber core""s end under an angle sufficiently small relative to the fiber axis can enter the fiber and is refracted according Snell""s law into a certain angle. It then hits the interface between fiber core and cladding and is, assuming the angle of incidence to the surface is sufficiently large, totally reflected back into the core. If no bends occur that exceed a critical curvature, the light cannot leave the fiber core and is thus guided through the fiber until it reaches the end. If the core is small, typically in the range of a few multiples of the wavelength of the radiation that is to be coupled into the fiber, the light guiding effect can be easier understood in terms of a wave guiding.
The fiber is an optical system wherein light propagation is possible only in distinct Eigenmodes. These modes can be excited by incident light and then propagated through the fiber. All light guiding effects in optical fibers share a common feature, the optical field is not completely confined to the fiber core. Slight parts of the radiation lap into the fiber cladding; known as an xe2x80x9cevanescent fieldxe2x80x9d to people skilled in the art. This evanescent field does not necessarily contribute to the damping of the fiber, but it can significantly influence the guiding and mode properties.
Since it is possible to couple into fibers the radiation of high power light sources, such as diodes and laser beams, one can think of applying specially manufactured illuminating fibers in a wide variety of applications.
Normally the goal of the fiber manufacturing process is to minimize the fiber""s intrinsic losses. Illuminating fibers show a different behavior than conventional fibers, because their optical loss is not usually as small as possible but well defined over the length of the fiber. This is realized by manufacturing the fiber in such a manner that a certain amount is coupled out of the fiber""s radiation guiding core and is diffused into the fiber cladding, from where it is scattered. The fiber cladding appears to be illuminated. Illuminating fibers of this simple kind can be manufactured in several ways.
For polymer cladded fibers, one method treats the fiber chemically while still uncoated so that the core""s surrounding becomes rough and thus diffuses a certain part of the light being totally reflected at the core/cladding interface. This method has several disadvantages. It is a very rough method and can only be slightly regulated, thus the illumination effect will vary strongly with the length of the fiber. The technique cannot be used with glass cladded fibers.
Another method utilizes the scattering effect of several substances added to the basic material from which the fiber is manufactured. This permits a very homogeneous doping of the fiber core. Similarly, the polymer cladding can contain a dopant material, from which parts of the evanescent field are scattered so the fiber appears illuminated. This method can produce a uniform diffusion, but does not permit a patterned diffusion along the fibers length.
Although uses for illuminating fibers are suggested in the prior art, few discuss the use of partially diffusing fibers as an economical means to achieve the desired end products.
U.S. Pat. No. 3,508,589 describes luminous textile products made luminous by incorporating optical fibers, which have been enhanced to xe2x80x9cfrustrate total reflectionxe2x80x9d. Methods discussed are the disruption of the internal reflecting surfaces by roughening the surface of the unsheathed core by etching, grit blasting or abrading. It would be economically advantageous be able to produce the patterned diffusive properties at the time the fiber was manufactured. The methods described could not be used for a glass cladded fiber.
Since state of the art fibers have uniform scattering rates according to their fabrication process, a necessary requirement to realize the above mentioned applications is a manufacturing method to produce long lengths of diffusing optical fibers with tailored properties, especially concerning their scatter rates.
U.S. Pat. No. 5,737,472 describes an optical fiber with multiple point lateral illumination. The method that the invention proposes is treating a fiber of length on the order of several meters, to produce an appearance of uniformity or quasi-continuous luminosity. The illumination is accomplished by numerous, closely spaced degradations on the fiber surface. The number and size of the degradation are a function of their distance from the illumination source. The degradations are obtained by sandblasting or attack by an aerosol solvent. The patent describes several methods of maintaining uniform illumination. These include changing the sandblaster characteristics as a function of fiber length and using a photocell to measured the intensity of perceived light at the sandblast site, which controls the speed of the fiber. In one embodiment of the invention, the optical fiber includes several treated areas separated by non-treated areas. The invention illustrates a spool-to-spool (fixed length) post-draw process and is not suitable for very long lengths of fibers.
U.S. Pat. No. 5,905,837 describes a method to controllably tap and distribute light propagating through an optical fiber. The invention comprises an optical fiber having multiple cross-sectional regions each having a different index of refraction. When light passing through the fiber reaches the interface where the refractive index changes, the light traversing the fiber is diverted out of the optical fiber through the side of the fiber. Refractive regions and reflective layers help to direct light out of the fibers. Prisms may advantageously be applied to the exit side of the fiber to focus the light for use. The invention primarily relies on dispersive elements in the optical fiber material, reflective elements and prisms. The reflective and prismatic elements are not derived from the fiber itself. It does not suggest the advantageous treatments of the cladding or combinations of core and cladding. The patent does not describe or suggest a continuous in-line operation integrated into the production of the fiber.
U.S. Pat. No. 5,781,679 describes an apparatus for tapping and dispersing light from an optical fiber. The invention comprises mirrors constructed from the optical fiber itself through a series of micro-cutting, masking, coating and refilling operations. Dispersive elements are added to the refilling material before it is placed within the cut region of the fiber. The cut and refilled regions act as a tap allowing diffused light to exit the optical fiber. The invention does not describe or suggest producing the light-dispersing fiber in a continuous process nor does it discuss the treatment of a fiber cladding for enhanced illumination effects.
U.S. Pat. No. 6,044,191 discloses a single-mode optical waveguide fiber having variations in properties that provide dispersion that varies along the length of the waveguide. One embodiment describes a core preform having sections of reduced diameter. Several techniques for changing the diameter of the core are heated stretching, grinding, polishing, etching, and laser ablation. The core is then overcladded. In another embodiment, the refractive index of the core is varied by irradiation or bombarding the waveguide with sub-atomic particles. The treatment is done after the fiber has passed out of the furnace and before it has received a polymer coating. The patent does not describe the advantage of using multimode fibers for illumination. The patent does not disclose the advantageous modification of both the core and cladding, and avoids treatments within the hot production zone.
PCT Application WO 99/23041, published May 14, 1999, discloses a fiber optic diffuser in which the scattering elements are generated by an optical damage process to the core of an optical fiber using pulsed lasers. Multiple small scattering centers are created in the core. The scattering centers consist of small regions of optically damaged core, which have the characteristic of scattering light. During manufacture of a diffuser, the irradiance distribution being created is measured using an emission source attached to the proximal end of the optical fiber opposite the diffuser end. Creation of scattering sites is detected by optical output detection at the distal end of the fiber. Although the patent does describe non-uniform or customized diffuser emission profiles, it does not describe a continuous in-line operation integrated into the production of the fiber. It does not suggest combination treatments of both the core and cladding in a single process.
PCT Application WO 00/79319, published Dec. 28, 2000, describes an optical fiber diffuser with pre-selected light intensity distributions along the length of the diffuser. Irradiating the fiber core with high power UV light through an amplitude mask or a phase mask preferably produces the scattering centers of the fiber diffusers. The method describes removing a buffer layer that coats the cladded core in areas where a proposed diffuser is to be created followed by a buffer recoat. The patent also teaches that multimode fibers are preferred over single mode to give significantly higher emitted light intensities. The invention does not suggest treatment of the cladding to enhance light diffusion. The patent does not disclose a method to produce commercially long lengths (in excess of several meters) of partially diffusing optical fibers.
All the prior art production of illuminating fibers is performed on relatively short lengths and generally requires secondary operations to make the fiber light diffusing. Prior art operations to produce partially diffusing optical fibers all occur outside the hot production zone. In view of the large-scale applications of partially diffusing optical fibers, there is still a need for a method to produce optical fibers having integrally formed light diffusers and in commercial lengths.
It is an object of the present invention to provide an apparatus to manufacture custom optical fibers having selected areas of light diffusion along their length.
It is another object of the present invention to provide a method to manufacture partially diffusing optical fibers through an inline continuous operation.
It is another object of the present invention to provide a process that is capable of producing partially diffusing optical fibers having significant lengths in an efficient and economical manner
It is yet another object of the present invention to provide an apparatus having several different means to enhance the light diffusive properties of an optical fiber in selected areas along a continuous length.
It is a further object of the present invention to provide a light diffusing optical fiber with controlled patterns of diffusion per length segment.
Briefly stated the present invention provides for an apparatus and method to manufacture optical fiber in a way that produces controlled and patterned diffusion of optical radiation along its length. The novelty of the described invention is that the patterns of diffusion are produced at the time the optical fiber is manufactured. The xe2x80x9cin-linexe2x80x9d manufacturing method avoids the need for post-production treatment of the fiber, which makes the process highly efficient and economical. Light diffusing optical fibers of significant length can be produced. Several manufacturing configurations to achieve the desired effects and their inclusion in the fiber production process are described. The processes can be configured to process optical fibers constructed from a wide variety of known glass, polymeric or other materials. The partially diffusing optical fibers of this invention have applications ranging from illuminated fabrics and toys to lighting systems and medical instruments. A distributed sensor comprising a light detector coupled to a partially diffusing fiber is also disclosed.
The above and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, (in which like reference numbers in different drawings designate the same elements.)